Seismic surveying



@et if, msm. .1. E. OWEN 2,216,452

` sEIsmIc SURVEYING Original Filed Nay 27. 1938 Y2 Sheets-She-et 1 /J pl a TL@ E- mi Z3 .A INVENTOR.

Oct. 1, 1940. J. E. OWEN SEISMIC SURVEYING' Original Filed May 27, .1938 2 Shams-Sheet 2 INVENTOR. BY www ATTORNEYS physical i'- fr: arch Corporation, New York, N. Y.,

a corporation of New Jersey Application May 27, 1938, Serial No. 210,349 Renewed July 2, 1940 This invention relates to seismic surveying.

In the reiiectionwave method oi' seismic surveying, the reection wave record is complicated by the eiects produce by surface waves generated at the explosion of the shot unless provision is made for elimination of such eiects from the record.

An object of this invention is a method of eliminating the objectionable effects 'of surface waves from a reiiection wave record.

The surface Waves above referred to are the waves commonly known as ground roll and are o! large amplitude and low frequency and travel out from the shot point along the surface of the earth. These waves are closely analogous to the surface waves known as Rayleigh waves which are a well-known type of wave produced as a result oi.' natural earth quak'es. The earth particles executing such a wave motion move over an elliptical path and the motion of a particle is retrograde with respect to the direction of propagation of the wave. l

According to the present-invention, two geophones are provided at the recording station,

2B and are arranged one in a vertical position and the other in a horizontal position with its axis pointed toward the shot point. The first or vertical geophone will respond only to Vertical motion while the second or horizontal geophone 3o will respond only to horizontal motion. The displacement of both geophones when actuated by ground roll is the vector sum of a horizontal and a vertical component of displacement 90 out of phase. Since the motion is elliptical, the two components of the displacement may be represented by z=B cos wt =A Sin wto where Since the vertical geophone responds only to the vertical component, its output will be proportional to A sin wt. Similarly, the output of the horizontal geophones will be proportional to B C08 u The output from the horizontal geophone is (Cl. Isl- 0.5)

e=A (Sinto-Ba (sin at) :oi-Bw) sin wt The effect oi the surface Waves is completely eliminated by making the combined output equal to zero which is accomplished by making A=Bo which inturn is very simply accomplished merely by adjusting the relative amplitudes of the two components by means of suitable controls on the recording amplifiers.

'I'he above-described balancing may be accomplished equally well by running the output from one of the geophones through an electrical circuit which performs an operation equivalent to integrating and then combining such altered output with the output of the other geophone. For example, by integrating B cos wt which represents the output of the horizontal geophcne. there is obtained the expression By combining the integrated output of the horizontal geophone with the output of the vertical geoplione, but out of phase, such combined output is expressed as follows:

e=A sin catsin wt fana) a t (A) The condition of balance obtained by making hat this frequency may be adjusted to the prelominating frequency of the ground roll merelyby varying the amplitudes of the two geophone outiuts. Usually, most of the energy contained in :round roll does reside in a band close to one prelominating frequency so that the above system vorks quite well. It is true, however, that ground 'oll is transient -in nature and for that reason does :arry energy in a large number of frequencies so hat to balance it out completely a more elaborate ystem must be used. In such more elaborate nystem. the output from one of the geophones is 'ed into a phase shifting network, which is so lesigned that it shifts the phase of practically all frequencies contained in a wave fed through it by mproximately 90. The altered output thus ob- `ained is then ycombined with the unaltered outiut of the second geophone and the relative amilitudes are adjusted to give a balance.

Other objects, novel features and advantages oi' yhis invention will become apparent from the folowing specification and accompanying drawings, wherein:

Figs. l, 2, 3 and 4 are diagrammatic illustrations f different embodiments of the apparatus of the nvention.

In the embodiment illustrated in Fig. 1, two :eophones il and i 2 are provided at the receiving :tation of which il is arranged in a horizontal )lane with its axis pointing toward the shot point rnd the geophone i2 is arranged in a vertical po- :itiorn The output of the geophone il is coniected through a voltage divider I3 across the nput of a vacuum tube amplifier ifi. The outiut circuit of the amplifier it is connected .hrough the transormer i5 with a differentiating retvforlr i@ consisting of a condenser il in combiiation with a bridged resistance i8, the output of yhe network being connected across the input of a .econd vacuum tube amplifier iii. The geophone 2 is connected through a voltage divider 2@ beyWeen the input terminals of a vacuum tube am nlifier 2 i. The output circuit o the vacuum tube implier iti is connected through a transformer i2 with the input circuit of a vacuum tube amxliiler The output circuit of the ampliiler iii ncludes a winding 2t which constitutes one part if the primary of a transformer 2t, while the outaut circuit of the amplifier Z3 includes a winding 1&5 which constitutes the remainder of the primary if said transformer. if the transformer is connected to a recording :alvanometer 255 of the type which makes but a ingle trace. The secondary winding of the ransiormer '535i is connected to a recording gal- 'anoxneter 2&3.

The system illustrated in Fig. 2 is similar to `hat illustrated in Fig. 1, except that the diereniatlng network it oi Fig. 1 is replaced by the ntegrating network i consisting of a resistance 8 bridged by a condenserl i '5.

Considering the differentiating network in Fig. the input voltage e1 (t) produces a current hrough the condenser i7 and resistance l 8 which nay be termed z (t) The reactance of the conienser i1 is Very large compared with the resistnce I8 so that the current i (t) is determined arimarily by the Value of the condenser i1, i. e., t will be approximately 90 out of phasewith the xciting voltage and will be proportional to freluency. The output voltage en (t) being equal to (t) R (R being the value of the resistance i8), vill. therefore, be also 90 out of phase with e1 (t) .nd will be proportional to frequency. In other The secondary winding 2l.

words, the voltage ea (t) will approximate the time derivative of the voltage e1 (t).

In the integrating network of Fig. 2, the values of I1 and i8 are so proportioned that for the frequency involved, the value of the resistance ll is high compared with the reactance of the con-l denser I'I. The current i (t) flowing through the circuit will then be very nearly in phase with the exciting voltage and approximately equal in' value (R being the value of resistance iS). The voltapproximately (C being the reactance oi the condenser I1).

In the system illustrated in Fig. 3, the output of the horizontal geophone 3i is connected through a phase sluiting network 33 to the input of the vacuum tube amplifier 34. This network is the same as that shown and described in an article by O.VJ. Zobel in the Bell System Technical Journal, volume VII, No. 3, page 51e. By properly choosing the constants o this network, it may be made to give a phase shift of. approximately ii for all frequencies above an arbitrarily assigned frequency which, for the purpose of this inven tion, may be set somewhere in the neighborhood of cycles. The phase shift is obtained, with a practically ilat frequency characteristic or, in other words, there is no appreciable frequency distortion. The output oi ampiiler 3ft is con nested through a transformer @Si and voltage divider 3S across the input of a vacuum tube am pliier tl having a winding in its output. vertical geophone 32 is coupled through. the inm termediary of the resistance 3ft to the input of e. vacuum tube ampliier iid. The output oi' the vacuum tube amplifier is connectedthrough the transformer di and voltage divider @E to the input of a vacuum tube amplier to. The output circuit of amplier t? includes a winding 38 which constitutes one part of the primary for the transf..

balance each other out and have no effect uponr the recorder. The amplifiers 37,' and t3 of Fig. 3 are likewise properly adjusted to have the same effect.

In the system illustrated in Fig. d, 5i is either a. displacement geophone or a velocity geophone arranged in such position as to be responsive only to horizontal motion while 52 is either a velocity geophone or an acceleration geophone arranged to be responsive only to vertical movement. The output from the geophone 5i is impressed through the voltage divider 53 upon the input of an amplifier 54 which may consist of one or more stages of amplification. The output from the geophone 52 is impressed through the voltamaesa same range of frequencies. Since velocity is the time derivative of displacement and conversely displacement is the integral of velocity, the outputs of these geophones after suitable adjustment of amplitude may be added directly together to obtain elimination of ground roll in the manner described. This modification has the advantage of simplifying the electrical circuit required in the recording channel by permitting the omission of the integrating or differentiating circuit. Also, in the system just described, the velocity type geophone may be paired with a geoplane the response of which is proportional to the acceleration o the earths motion. In this case, thel same effect will .be obtained, since acceleration is the time derivative of displacement and conversely velocity is the integral of acceleration.

It isof course understood that variousmodifications may be made in the apparatus above described, without in any way departing from the spirit of the invention as defined in the appended claims. Furthermore, it is intended that the claims shall cover all such modifications as fall within the scope of their terminology. 'i'he term geophone is used to cover all types of devices which may be used in seismic surveying for translating earth vibrations into electrical waves.

I claim:

1. 'Ihe method of seismic surveying which comprises creating a source of seismic waves at the earths surface, separately detecting the vertical and horizontal components of the resulting earth motion at a point on the earths surface, said earth motion being the resultant of motions produced by surface waves, by refracted waves and by reflected waves, converting each of said detected earth motion components into a corresponding electrical wave, adjusting said electrical waves to bring the surface wave components thereof into proper relationship subf'lf'f stantially to cancel each other when combined.

.A combining the adjusted electrical waves and recording the resultant wave form.

2. The method of seismic surveying which com prises creating a source of seismic waves at the earths surface, separately detecting the vertical and horizontal components of the resulting earth motion at a point on the earths surface, said earth motion being the resultant of motions produced by surface waves, by refracted waves and by reected waves, converting each of said detected earth motion componentsinto a corresponding electrical wave, adjusting said electrical waves to bring the electrical wave component corresponding to the horizontal component of the surface wave earth motion into proper relation with the electrical wave component corresponding to the vertical component of the surface wave earth motion substantially to cancel each other when combined, combining the adjusted electrical waves and recording the resultant wave form.

3. The method of seismic surveying which comprises creating a source of seismic waves at the earths surface, separately detecting the vertical and horizontal components of the resulting earth motion at a point on the earths surface, said earth motion being the resultant of motions produced by surface waves. by refracted waves and by reected waves, converting said separately detected earth motion components into corresponding electrical waves of which one electrical wave is the time derivative ofthe other electrical wave, adjusting said electrical waves to equalize the amplitude of the surface wave components thereof, combining the adjusted electrical waves in such relation as to cancel said surface wave components and recording the resultant wave form.

v4. The method of seismic surveying which comprises creating a source of seismic waves at the earths surface, separately detecting the vertical and horizontal components of the resulting earth motion at a point on the earths surface, said earth motion being the resultant of motions pro.

duced by surface waves, by refracted waves and by reflectedl waves, converting each of said detectedearth motion components into a corresponding electrical wave, altering one electrical wave to make the surface wave components of both electrical waves proportional to the same trigonometric function but of dierent sign., effecting adjustment of the altered and unaltered electrical waves to bring their surface wave components to the same amplitude, combining the adjusted electrical waves and recording the resultant wave form.

5. The method of seismic surveying which comprises creating a source of seismic waves at the earths surface, separately detecting the vertical' to bring their surface wave components to the same amplitude, combining the adjusted electrical waves and recording the resultant wave' form. y

6. The method 'of seismic surveying which comprises creating a source of seismic waves at the -f earths surface, separately detecting the vertical and horizontal components of the resulting earth motion at a point on the earths surface, said earth motion being the resultant of motions produced by surface waves, by refracted waves and by reflected waves, converting each of said detected earth motion components into a corresponding electrical wave, subjecting one electrical wave to integration to make the surface wave components of both electrical waves proportional t'othe same trigonometric function but of different sign, eecting adjustment of the altered and unaltered electrical waves to bring their surface wave components to the same amplitude, combining the adjusted electrical waves and recording the resultant wave form.

7. The method of seismic surveying which comprises creating a source.of seismic waves at the earths surface, separately detecting the vertical and horizontal components of the resulting earth motion at a point on-the earths surface, said earth motion being the resultant of motions produced by surface waves, by refracted waves and by reflected waves, converting each of said detected earth motion components intol a corresponding electrical wave, altering the electrical wave corresponding to the horizontal component of the surface wave earth motion to make the surface wave components o! bothv electrical waves proportional to the same trigonometric function but of different sign, eifecting adjustment of the altered and unaltered electrical waves to bring their surface wave components to the same amplitude, combining the adjusted electrical waves and recording the resultant wave formed.

8. The method of seismic surveying which comprises creating a source of seismic. waves at the earths surface, separately detecting the vertical and horizontal components of the resulting earth motion at a point on the earths surface, said earth motion being the resultant of motions produced by surface waves, by refracted waves and by reflected waves, converting each of said detected earth motion components into a corresponding electrical wave, subjecting the electrical wave corresponding to the horizontal component to the surface wave earth motion to diiferentiation to make the surface wave components of both electrical waves proportional to the same trigonometric function but of different sign, effecting adjustment of the altered and unaltered electrical waves to bring their surface wave components to the same amplitude, combining the adjusted electrical waves and recording. the resultant wave form.

9. The method of seismic surveying which comprises creating a source of seismic waves at the earths surface, separately detecting the vertical and horizontal components of the resulting earth motion at a point on the earths surface, said earth motion being the resultant of motions produced by surface waves, by refracted waves and by reflected waves, converting each of said detected earth motion components into a corresponding electrical wave, subjecting the electrical wave corresponding to the horizontal component oi' the surface wave earth motion to integration to make the surface wave components of both electrical waves proportional to the same trigonometric function but of different sign, eifecting adjustment of the altered and unaltered electrical waves to bring their surface wave components to the same amplitude, combining the adjusted electrical waves and recording the resultant wave form.

10. The method of seismic surveying which comprises creating a source of seismic waves at the earths surface, separately detecting the vertical and horizontal components of the resulting earth motion at a point on the earths surface, said earth motion being the resultant of motions produced by surface waves. by refracted waves and `by reflected Waves, converting each of said detected earth motion components into a corresponding electrical wave, adjusting said electrical waves to make the surface wave component of one electrical wave equal in amplitude but 180 out of phase with the surface Wave component of the other electrical wave, combining the adjusted electrical waves and recording the resultant wave form.

JOHN E. OWEN. 

